Coupling networks are well known in the art and are used in applications from audio to radars and wire taps. They are used to interconnect devices or subsystems and their functions. Coupling networks have applications in the 1) isolation of dc components; 2) shaping the amplitude or phase-angle transfer function; 3) impedance matching; and 4) real time direct control of causal feedback characteristics. In all cases, the coupler design depends not only upon the parameters of the coupling network, but also upon the specific circuit into which the coupling structure is imbedded.
In a typical radio frequency (RF) application, the directional coupler is used in the RF transmitter Automatic Level Control (ALC) circuit. The transmitter ALC senses the power level being produced by the RF Power Amplifier (RFPA) and adjusts the bias of the RFPA to set the RF power output to a desired level. A typical portable's ALC consists of the coupler which "lightly couples" RF energy from the main RF transmission line (between the RFPA and antenna), a rectifier which rectifies the coupled RF energy producing a dc voltage proportional to the RF power level, and a comparator which compares the rectified dc voltage to a reference voltage. The comparator's dc output is connected to the RFPA bias transistors and is adjusted to re-bias the RFPA to the proper power level. This application requires that the coupler induce low insertion loss in the main transmission path between RFPA and the antenna, while at the same time coupling enough energy into the ALC to be properly processed.
Moreover, the typical coupling structure can utilize broad edge or co-planar, microstrip or strip-line transmission structures. The prior art coplanar structure shown in FIG. 1 utilizes paired parallel transmission lines in the same horizontal (edge-to-edge) plane and functions predominantly as an inductive coupling (magnetic field) structure. As seen in FIG. 2, the broad edge coupling structure positions the coupling transmission line such that the width of the coupling transmission line "overlaps" the main transmission line along the vertical axis. The broad edge topology functions predominantly as a capacitive coupling (electric field) structure. In both the broad edge and coplanar topology, the primary RF transmission line is separated from the coupling transmission line by the appropriate distance to achieve the desired level of electro-magnetic coupling factor or coupling coefficient. A typical coupling factor of 20 dB will provide for proper ALC functionality with minimal insertion loss. For example, if 1 watt is being transmitted through the main transmission line, then 0.01 watt is being coupled onto the parallel transmission line. By definition, the microstrip transmission line structure uses air as the dielectric medium above the paired transmission lines. As seen in FIG. 3, the strip-line transmission structure is buried into any given dielectric material with ground planes "sandwiching" the dielectric encased coupler structure.
Multiple coupled line structures can be used to effect a bi-directional forward and/or reverse power coupling structure. FIG. 3 shows an example of a coupled line structure utilizing three lines. Forward and reverse (bi-directional) power coupling structures function as a dual mode topology to ensure both proper RFPA power levels and to cutback the RFPA power in the event of a fault in the transmitter system. The forward coupling transmission line senses the forward RF energy (power to the antenna); therefore, the dc voltage generated from the rectified forward coupled energy is directly proportional to the forward traveling RF energy. In the event of a transmitter system fault ( i.e. an open in the antenna) the RF energy is reflected back from the antenna towards the RFPA. Serious or catastrophic RFPA damage can result if the RFPA continues to transmit full power under a fault condition.
To protect against RFPA damage, a reverse power (power to the RFPA) cutback circuit using a reverse power coupling transmission line structure can be added to sense the level of reflected RF energy. As with the forward ALC topology, the reverse RF energy is rectified and the dc voltage from the reverse coupled structure is directly proportional to the reverse traveling RF energy. The bi-directional coupling factors can be independently set by variations in the geometries of the individual coupling transmission line and can be predicted using standard field solver techniques.
The electrical differentiation between the forward and reverse structure depends upon the placement of the termination impedances, control of the transmission line characteristic impedance and coupler separation from the primary transmission line. Directivity is a measure of the bi-directional coupler differentiation. The algebraic difference in dB of the forward and reverse coupling coefficients for any fixed structure is defined as the directivity of that structure. A 20 dB directivity factor is considered acceptable for a bi-directional coupler. This means that if a forward coupling structure measures a forward coupling coefficient of 20 dB, that the reverse coupling coefficient for that same structure will measure 40 dB; thus the directivity for the forward coupler is said to be 20 dB. The lower the directivity factor, the less sensitivity the ALC will have in differentiating between forward and reverse RF energies.
The coupler structure is critical to proper ALC functionality and protection of the RFPA. The bi-directional coupler design requires minimizing manufacturing tolerance effects and closed loop variations. PC board manufacturing variations in transmission line widths and inner-layer dielectric thicknesses have usually prevented the coupler from being embedded into the RF PCB (printed circuit board) itself. Historically, high performance bi-directional couplers have been fabricated on a substrate such as alumina, with thin film processes and their tight tolerance capability defining the coupler geometries to achieve a controlled 20 dB coupling coefficient with greater than 20 dB of directivity. Although the modularized approach to implementing the coupler is effective, it adds cost and process steps that could otherwise be eliminated if the coupler were to be embedded into the PCB itself. To achieve high reliability coupler performance with existing PCB make tolerances of .+-.2 mil gaussian width distributions requires an innovative design approach. Therefore, the need exists for embedded PCB coupler structure which achieves an acceptable coupling coefficient and directivity.